Initial development of wireless communications technology in the 3rd-Generation Partnership Project (3GPP) and other industry groups is underway for so-called fifth-generation wireless networks, often referred to as “5G.” Among other possible developments is the use in 5G of directed beams for some or all of the communications between wireless devices and the radio access network. These directed beams may be formed by a network node for transmitting to wireless devices (i.e., for downlink transmissions), or for receiving transmissions from a wireless device (i.e., for uplink reception), or both. As a consequence, the cell-centric approach to mobility and other wireless network aspects may be replaced or augmented with a beam-centric approach.
One important issue that arises with the use of beams to serve wireless devices is mobility, i.e., the handling of wireless devices as they move from one location to another, such that they need to be served by different or differently-directed beams. Note that with a beam-based approach, mobility can be between different beams belonging to the same network node, or between beams belonging to two different network nodes. With a beam-centric approach to beam mobility, the wireless device (often referred to as a “user equipment,” or “UE,” in 3GPP documentation) may be unaware of whether or not the beams belong to the same network node.
In some cases, relatively narrow beams may be used to minimize interference, and to provide the best link conditions. However, the use of narrow beams also implies that the link may deteriorate rapidly outside the optimal beam configuration, especially in circumstances involving sudden shadowing, a fast moving user, and/or strong interfering beams. This may result in short time windows for performing a beam switch, i.e., a handover from one beam to another. In some situations, such as when a rapidly moving wireless device suddenly turns around a corner, for example, the beam switch can be quite time-critical, since the quality of the serving beam may drop very rapidly.
The time window for doing a beam switch can be defined as the time from when the serving beam is no longer the optimal beam to when it becomes too weak for decoding of control or data signaling. If the serving beam is lost, the wireless device will first go into an out-of-synch (OOS) condition, and eventually will declare a radio link failure (RLF) if no new serving beam is found. An RLF triggers a new search and acquisition process, which can interrupt and degrade ongoing voice and/or data sessions. Particularly short beam-switch time windows may be avoided to some degree in the cell planning process, by trying to avoid beam shadowing at least in areas where users are moving fast. However, a combination of narrow beamwidth and a fast moving user, in a direction tangential to the beam's primary axis, will nevertheless create short beam-switch time windows.
In this context, then, it will be appreciated that the system should preferably be designed to provide a maximum time for performing a beam switch that is less than the minimum beam-switching time window, or at least to ensure that the time needed for beam switching is almost always less than the beam-switching time window. The requirements driving the design of mechanisms for ensuring that beam-switching is quick enough will often be driven by mobility between downlink beams, since downlink measurement results are not available to the network until after a signaling delay for reporting the measurements.